26 32 D:\编程\2.1.cpp [Error] 'endl' was not declared in this scope

This error message suggests that the program is trying to use the `endl` keyword, which is typically used in C++ to insert a newline character into the output stream, but the keyword has not been declared or defined in the current scope.

To fix this error, you can include the `<iostream>` header file at the beginning of the program, which contains the definition for `endl`, or you can use the newline character `\n` instead of `endl`.

For example, you can modify your code as follows:

#include <iostream>

using namespace std;

int main() {
    cout << "Hello, world!" << endl; // or replace endl with \n
    return 0;
}

This should resolve the error and allow the program to compile successfully.